"I have spent a lot of time arguing that the theory of group selection is not the stupid, pernicious doctrine that many biologists once claimed it to be. The theory is not just conceptually coherent; there are adaptations out there in nature (like reduced virulence in some viruses) that evolved because there was group selection."

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Another way to test hypotheses about adaptation is to consider trait variation across a group of species instead of focusing on the trait of a single species. Rather than seeking to explain why polar bears have fur of a certain thickness, one tries to explain why bears in colder climates have thicker fur than bears in warmer climates. The former problem is hard to solve, since it is hard to say exactly what fur thickness polar bears should have if natural selection guided the evolution of that trait.

— Elliott Sober